Conventional reference electrodes have limited lifetimes due to contamination of the internal reference elements by the sample or by fouling of the restriction devices which are used to control the flow of reference electrolyte from the interior of the electrode into the sample. For example, typical restriction devices used in electrodes include porous glass or plastic frits, sleeve joints, ion exchange membranes, etc. Such conventional reference electrodes exhibit large offsets, unpredictable temperature coefficients and/or unstable potential readings after short periods of usage.
In conventional electrodes, the liquid sample can diffuse past the restriction device and into the reference electrolyte (which may be a liquid, gel, or polymer substance). This diffusion cannot be completely reversed, and eventually sample constituents will reach the reference element, potentially poisoning it.
Some manufacturers have been able to slow down contamination of the internal reference elements by using double junction references, allowing the reference electrolyte to be replaced periodically. However, this is a cumbersome process and does not address the problem of fouling of the restriction device.
Free flowing reference outlets can be used instead of restriction devices to prevent fouling. With a free flowing reference outlet the reference electrolyte and sample solution form an interface where ions are free to flow between the two solutions. There are no physical barriers, except for the limited exposed surface area of the junction, to impede the flow of electrolyte into the sample (and vice versa). Unrestricted flow offers the best possible means of minimizing reference junction potentials caused by fouling, but can lead to problems of excessive reference electrolyte leakage, thus contaminating the sample.
The Orion brand "Sure Flow" device (and others of similar type) uses a ground glass seat and seal in a reference electrode. The seal consists of the glass reference bulb. In normal operation of such device the liquid electrolyte seeps through this seal. The glass reference is prone to clogging and contamination by the sample being tested. To renew and clean the reference, the user presses the top of the electrode to cause the bulb to separate from the seat. This allows the low viscosity electrolyte to flow through the open seal (by gravity feed), flushing the surface of the bulb clear of contamination. This is a very uncontrolled process for cleaning the glass bulb. Not only can the surfaces not be washed clean at times, but the entire reservoir of electrolyte can be quickly used for this cleaning function. Thus, the entire supply of electrolyte can be rapidly exhausted.
Other problems which can be encountered with reference electrodes of the type which include a hollow tube to supply liquid electrolyte. For example, the tube can become pinched, broken or clogged. A high leakage rate through the tube has also been observed. Also, problems interfering with flow of electrolyte can be caused by failure of the connectors used.
There has not heretofore been provided a reference electrode having the advantages provided by the present invention.